This invention relates to a double cylinder type shock absorber having an oil chamber and a gas chamber, and more particularly to a shock absorber capable of effectively preventing a gas in a gas chamber from leaking therefrom.
In a double cylinder type shock absorber using an oil and a gas and consisting of a combination of a hydraulic means composed of a piston and an oil cylinder and a gas tube provided concentrically on the outer side of the hydraulic means to form a gas chamber therebetween, it is necessary that a means for preventing the leakage of a pressure gas in addition to a seal means for preventing the leakage of a pressure oil be devised. FIG. 1 shows an example of a double cylinder type shock absorber, which is disclosed in Japanese Published Unexamined Utility Model Application No. 1979-53193, and which is provided with seal means of the above-mentioned kind for preventing the leakage of a pressure oil and a pressure gas. This shock absorber has the following constructional elements. Namely, it is provided with a smaller-diameter oil cylinder 1 and a larger-diameter gas tube 2, which are joined together in a concentrically arranged state to form a gas chamber 3 therebetween; a piston (not shown) mounted fixedly on an end portion of a rod 4 and disposed in the oil cylinder 1 in such a manner that the piston can be moved in the axial direction thereof; a bearing 5 adapted to guide the reciprocating movements of the rod 4, fixed to an end portion of the oil cylinder 1, and forming an oil chamber 6 in cooperation with the oil cylinder 1 and rod 4; an oil seal cover 9 supported on an end portion of the gas tube 2; an oil seal 10 consisting of a rubber-like elastic material, disposed on the inner side of the oil seal cover 9, and adapted to preventing an oil from leaking outside and the external dust from entering an inner space on the side of the bearing 5; a check valve 11 consisting of a rubber-like elastic material, having a check valve lip 12 fixed to a washer 13, having a substantially L-shaped cross-section, and disposed in a position adjacent to the oil seal 10; a conical circumferential surface 15 formed on the portion of the bearing 5 which is on the side of the oil seal with the check valve lip 12 resiliently contacting the conical circumferential surface 15 to form an inner hollow chamber 16 on the side of the rod 4 and an outer hollow chamber 17 on the side of the oil seal cover 9; a coiled spring 18 disposed in the outer hollow chamber 17 and adapted to urge the check valve 11 toward the oil seal 10; and a communication bore 19 formed suitably in the vicinity of the outer circumferential surface of the bearing 5 so as to communicate the outer hollow chamber 17 with the gas chamber 3.
When the piston and rod 4 in the above shock absorber are moved upward, the pressure in the oil chamber 6 increases, so that a part of the oil flows into the inner hollow chamber 16 through a clearance between the bearing 5 and rod 4. At this time, the oil seal 10 prevents the oil from leaking outside. Consequently, the check valve lip 12 is expanded, so that the oil flows into the outer hollow chamber 17. The oil then flows through the communication bore 19 into the gas chamber 3 to be mixed with the oil existing in a lower portion thereof. When the piston and rod 4 are moved downward, the pressure in the oil chamber 6 decreases, and the lip 12 of the check valve 11 is brought into press contact with the conical circumferential surface of the bearing 5. Accordingly, the pressure gas in the gas chamber 3 and outer hollow chamber 17 is prevented from flowing into the inner hollow chamber 16.
In the above shock absorber, the oil seal 10 and check valve 11 are supported on the coiled spring 18. Therefore, the actions of the oil seal 10, which is drawn downward due to the friction between the rod 4 and oil seal 10, especially, during a downward stroke of the rod 4, having influence on the contacting condition of the check valve lip 12 and the conical circumferential surface 15 of the bearing 5. There is the possibility in such a shock absorber that the pressure gas in the gas chamber 3 leaks into the inner hollow chamber 16 and flows into the oil chamber 6 to be mixed with the oil, or to leak outside through the sliding portions of the oil seal 10 and rod 4.